scanner: support "since" attribute for enum entries

This was already in the DTD but not supported by the scanner.

The check for ever-increasing "since" tags is not strictly required for enum
entries as we control the binary value. But it keeps the xml file in
good order, preventing things like:

      <entry name="first" value="…" />
      <entry name="second" value="…" since="3"/>
      <entry name="third" value="…" since="2"/>
      <entry name="fourth" value="…" since="3"/>

If this is undesirable in the future the check can be removed without
side-effects.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>